abstracts: geology 93 



Oligocene. It is shown in this paper that the sedimentation of Eocene 

 time was interrupted during several intervals, which were of consider- 

 able duration in terms of organic evolution. Such intervals occurred 

 between the Midway or basal Eocene deposits and the overlying 

 Wilcox group and between the Wilcox and the Claiborne. 



It is concluded that the strand line migrated back and forth over 

 the Mississippi embayment several times during the period represented 

 by the older Tertiary deposits. R. W. S. 



GEOLOGY.— The Willow Creek district, Alaska. S. R. Capps. U. S. 

 Geological Survey Bulletin No. 607. Pp. 86, with maps, sec- 

 tions, and illustrations. 1915. 



The geologic formations exposed consist of mica schists, possibly 

 of Paleozoic age; quartz diorites and gneisses, probably Mesozoic; 

 Eocene arkoses, conglomerates, shales, and sandstone, with some 

 interbedded basaltic lava flows; and Quarternary glacial deposits and 

 recent stream gravels. The Tertiary beds are somewhat folded, 

 but have prevailing dips of 20° to 50° to the southward. 



Gold quartz veins fill fissures in the quartz diorite, and occur in 

 two predominant sets, one striking northwest, and the other north- 

 east. The prevailing dip is 30° to 50° to the westward. The veins 

 show little surface oxidation and no secondary enrichment and prom- 

 ise to maintain the same characters with depth that they display 

 near the surface. 



Some gold placer deposits have been worked, but most of the con- 

 centrations of placer gold that must have resulted from the erosion 

 of the gold-bearing quartz veins were swept away and scattered by 

 the vigorous glaciers which occupied the valleys during the height of 

 the Quarternary ice invasion. S. R. C. 



GEOLOGY.— The Ellamar district, Alaska. S. R. Capps and B. L. 



Johnson. U. S. Geological Survey Bulletin No. 605. Pp. 125, 



with maps, sections, and illustrations. 1915. 

 The rocks exposed include the Valdez group of interbedded slates 

 and graywackes, possibly of Paleozoic age, and the Orca group, possibly 

 Mesozoic, including in ascending order: (1) fine black slates; (2) slates 

 and graywackes; (3) ellipsoidal lavas and massive diabase flows with 

 some interbedded sediments; (4) conglomerates and sandstones; and 

 (5) another series of slates and graywackes. The dominant structural 

 trend is northwest-southeast, and the beds have prevailing dips to 



